3. slim, slender, skinny, lank, scrawny. Thin,gaunt,lean,spare agree in referring to one having little flesh. Thin applies often to one in an unnaturally reduced state, as from sickness, overwork, lack of food, or the like: a thin, dirty little waif.Gaunt suggests the angularity of bones prominently displayed in a thin face and body: to look ill and gaunt.Lean usually applies to a person or animal that is naturally thin: looking lean but healthy after an outdoor vacation.Spare implies a muscular leanness with no diminution of vitality: Lincoln was spare in body.5. meager. 8. weak.

These our actors ... were all Spirits, and Are melted into Ayre, into thin Ayre. [Shakespeare, "The Tempest," IV.i.150, 1610]

Thin-skinned is attested from 1590s; the figurative sense of "touchy" is from 1670s.

v.

Old English þynnian "to make thin" (cf. German dünnen, Dutch dunnen), from thin (adj.). Intransitive sense of "to become less numerous" is attested from 1743; that of "to become thinner" is recorded from 1804. Related: Thinned; thinning.